Fiber-optic sensors in a rosette or rosette-like pattern for structure monitoring

ABSTRACT

An apparatus, and related method, relates generally to a fiber-optic sensing system. In such a system, fiber-optic sensors are in a rosette or rosette-like pattern. An optical circulator is coupled to receive a light signal from a broadband light source, to provide the light signal to the fiber-optic sensors, and to receive a returned optical signal from the fiber-optic sensors. A spectral engine is coupled to the optical circulator to receive the returned optical signal and configured to provide an output signal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This nonprovisional application is a divisional application of, andhereby claims priority to, pending U.S. patent application Ser. No.16/223,652, filed Dec. 18, 2018, which is a divisional application ofU.S. patent application Ser. No. 15/458,311, filed Mar. 14, 2017, whichis a continuation-in-part of U.S. patent application Ser. No.14/814,355, filed Jul. 30, 2015 (now U.S. Pat. No. 10,033,153), whichclaims benefit of priority to Provisional Patent Application Nos.62/062,429, filed Oct. 10, 2014, and 62/031,790, filed Jul. 31, 2014.U.S. patent application Ser. No. 15/458,311, filed Mar. 14, 2017,further claims the benefit of priority to U.S. Provisional PatentApplication No. 62/310,664, filed Mar. 18, 2016. The entirety of eachand all of the aforementioned provisional and nonprovisionalapplications are hereby incorporated by reference herein for allpurposes and to the extent same is consistent herewith.

FIELD OF THE INVENTION

The following description relates to structure monitoring. Moreparticularly, the following description relates to structure monitoringusing fiber-optic sensors in a rosette or rosette-like pattern.

INTRODUCTION

For real-time structural monitoring, conventional instrumentation maynot have sufficient performance, deployment capabilities, and/or otherlimitations. These one or more limitations may be a barrier toaddressing some monitoring applications. Along those lines, the“Internet of Things” or “IoT” has facilitated wide deployments ofsensors for sensed inputs sent over the Internet and/or another networkfor monitoring, as well as responding to detected events found throughsuch monitoring.

BRIEF SUMMARY

An apparatus relates generally to a fiber-optic sensing system. In sucha system, fiber-optic sensors are in a rosette or rosette-like pattern.An optical circulator is coupled to receive a light signal from abroadband light source, to provide the light signal to the fiber-opticsensors, and to receive a returned optical signal from the fiber-opticsensors. A spectral engine is coupled to the optical circulator toreceive the returned optical signal and configured to provide an outputsignal.

A method relates generally to sensing. In such a method, fiber-opticsensors are attached in a rosette or rosette-like pattern to astructure. A light signal is generated with a broadband light source.The light signal is received by an optical circulator. The light signalis provided from the optical circulator to the fiber-optic sensors. Areturned optical signal from the fiber-optic sensors is received by theoptical circulator. The returned optical signal is provided from theoptical circulator to a spectral engine to provide an output signal.

A method relates generally to a multi-sensing system. In such a method,a fiber-optic strain sensing system having a broadband light sourcecoupled to a controller is operated. A strain and a direction thereof ofa damage induced anomaly in a structure is detected. A laser-basedacoustic-emission sensing system is triggered responsive to thedetecting of the damage induced anomaly by the fiber-optic strainsensing system. Wavelength tracking logistics are provided formonitoring the damage induced anomaly.

Other features will be recognized from consideration of the DetailedDescription and Claims, which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

Accompanying drawings show exemplary apparatus(es) and/or method(s).However, the accompanying drawings should not be taken to limit thescope of the claims, but are for explanation and understanding only.

FIG. 1 is a block diagram depicting an exemplary fiber-optic strain(“FOS”) system.

FIG. 2 is a block diagram depicting another exemplary FOS system.

FIG. 3 is a block diagram depicting an exemplary rosette or rosette-likepattern for FBG fiber-optic sensors of FIGS. 1 and 2.

FIG. 4 is a block diagram depicting an exemplary wireless network.

FIG. 5 is a flow diagram depicting an exemplary fiber-optic strainsensing flow.

FIG. 6 is a perspective side view depicting an exemplary optical fiberand a corresponding reflected signal.

FIG. 7-1 is a block diagram of a perspective view depicting an exemplarypatch attached to a host structure.

FIG. 7-2 is a block diagram of a cross-sectional view along A-A of thepatch of FIG. 7-1.

FIG. 8 is a block diagram depicting an exemplary acoustic emission(“AE”) and FOS system.

FIG. 9-1 is a block diagram depicting the exemplary AE and FOS system ofFIG. 8 optionally coupled to a Composite Overwrapped Pressure Vessel(“COPV”).

FIG. 9-2 is a block diagram depicting the exemplary AE and FOS system ofFIG. 8 coupled to COPV.

FIG. 10-1 is a block diagram depicting an exemplary single-channel lasertracking-based fiber optic voltage conditioning (“FOVC”) system.

FIG. 10-2 is a block diagram depicting an exemplary multichannel FOVCsystem.

FIG. 11 is a block diagram depicting an exemplary self-poweredcontrol/detection system.

FIG. 12 is a block diagram depicting an exemplary wavelength trackingsystem using fiber optic voltage conditioning for AE sensing.

FIG. 13 is a flow diagram depicting an exemplary wavelength trackingflow.

FIG. 14 is a flow diagram depicting an exemplary damage induced anomalydetecting and monitoring flow for a multi-sensing system.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a more thorough description of the specific examples describedherein. It should be apparent, however, to one skilled in the art, thatone or more other examples and/or variations of these examples may bepracticed without all the specific details given below. In otherinstances, well known features have not been described in detail so asnot to obscure the description of the examples herein. For ease ofillustration, the same number labels are used in different diagrams torefer to the same items; however, in alternative examples the items maybe different.

Exemplary apparatus(es) and/or method(s) are described herein. It shouldbe understood that the word “exemplary” is used herein to mean “servingas an example, instance, or illustration.” Any example or featuredescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other examples or features.

One or more aspects relate generally to a multifunctional photonicsensor for monitoring. More particularly, one or more aspects relate toan integrated photonics sensing system capable of simultaneouslymeasuring strain/vibration (“strain”) and detecting damage. Examples oftypes of detectable damage include cracking, corroding, and/ordisbonding. Because such integrated photonics sensing systems are highlyportable, such integrated photonics sensing systems may be located ingenerally inaccessible areas of structures and/or generally inaccessiblestructures. Moreover, structures that can be monitored include withoutlimitation bolted, riveted and other types of fastened joints orinterfaces between two or more elements of a structure.

Along those lines, multifunctional integrated sensors with interrogationsystems may be used to lower cost and/or provide more effectivestructural health monitoring of long-duration structures, such asbuildings, bridges, long-duration space ships and other spacestructures, as well as other structures. The miniaturized, lightweight,and compact integrated photonics sensors described herein may be used tolocate damaged areas and/or detect probable failure zones with accurateassessment. An example of integrated photonics sensing system describedherein is capable of simultaneously measuring different types of strainand detecting damage.

Sensor technology described herein may provide one or more advantages orfeatures over conventional sensor technology. For example, sensortechnology described herein may be compact, lightweight, and powerefficient. In an example, a fiber-optic sensor technology describedherein may have: a volume footprint of approximately 0.5 mm×2 mm×5 mm orless; a weight of approximately a few grams; and a power consumption ofapproximately a few Watts. Furthermore, such sensor technology describedherein may be more sensitive to detection of damage and/or strain thanconventional sensor technology.

An aspect of this sensor technology is a fiber-optic sensor in a rosetteor rosette-like pattern. Along those lines, an acousto-ultrasonicfiber-based sensor, such as for example a Fiber Bragg grating (“FGB”)sensor, can be interrogated by a compact, wireless, battery poweredfiber-optic interrogator for strain and/or damage. Moreover, such FBGsensors may be coupled for detection static and dynamic fields such astemperature, strain, pressure, and acoustic waves, namely acousticemission (“AE”).

FBG sensors are wavelength-encoded, namely a Bragg wavelength, whichmakes FBG sensors self-referencing. Along those lines, FBG sensors maybe independent of fluctuating light levels and other optical noisesources. This wavelength-encoding property is also convenient formultiplexing. Along those lines, multiple FBG sensors may be used toprovide a distributed network for sensing.

Described below in additional detail is a capability of a rosette orrosette-like array pattern (“fiber-optic rosette”) of fiber-opticsensors which may be used to measure strain and/or detect damage. Afiber-optic rosette may be field deployed with a multichannel wirelessinterrogator (“interrogator node”) to allow data to be remotelyrecorded, analyzed, and displayed for visualization and large scaledamage prognostics. Along those lines, a fiber-optic rosette may be usedwith Cloud storage and/or Cloud computing for storage and/or analysis(“Cloud storage/computing”) of such data obtained.

A fiber-optic rosette may optionally be coupled with continuousmonitoring using an AE sensing system. This coupling of a fiber-opticrosette with an AE sensing system provides ability to identify afailure's driving mechanism by correlating different types of defectdata associated with process variables, such as load, strain,temperature, and/or pressure. Such AE sensing may be used for detectingsignals of micro and macro cracking and/or leaking.

FBG sensors are suitable for measuring static and dynamic fields such astemperature, strain, pressure, and acoustic waves. FBG interrogation asdescribed herein permits detection of a sub-microstrain resolution whilesimultaneously monitoring dynamic response, such as dynamic loads andstress waves, with high sensitivity and reproducibility.

When a damage site initiates or grows in a structure or material, astress field changes around such damage site which leads to generationof elastic stress waves. These elastic stress waves may travel throughsuch a structure or material to one or more AE sensors. Such AE sensorsmay be mounted on such structure or material to convert elastic stresswaves of a disturbance, such as due to a damage site, into electricalsignals. By analyzing such electrical signals, such as for a signal timeor arrival time for example, AE may be used for monitoring civilinfrastructure, pressure vessels, holding tanks, and/or other structuresto detect crack formation and growth, corrosion, and/or leaks, asapplicable. Such AE sensors may be FBG sensors as described herein in afiber-optic rosette.

FBG sensors are suitable for measuring static and dynamic fields, suchas temperature, strain, pressure, and/or acoustic waves. Sensedinformation from FBG sensors is wavelength-encoded. Along those lines,FBG sensors are self-referencing, rendering FBG sensors generallyindependent of fluctuating light levels and other optical noise sources.This wavelength-encoding property of FBG sensors offers convenientmultiplexing, such as time division multiplexing (“TDM”) or wavelengthdivision multiplexing (“WDM”), along a single optical fiber for makinghighly localized strain, temperature, and/or stress wave measurementsfor condition-based monitoring over a distributed area. For structuralhealth monitoring (“SHM”), FBG sensors can be used for both strain-basedload monitoring and AE-based damage detection.

As described below in additional detail, a fiber-optic rosette may beused for strain-based load monitoring. FBG sensors of such fiber-opticrosette may perform “double-duty” by also being used as AE sensors.These fiber-optic rosette sensors may be used with a miniaturizedinterrogation device, including a stand-alone compact multichannelfiber-optic strain/AE interrogator with wireless data acquisitioncapability for strain and damage monitoring.

In an example, a fiber-optic rosette and interrogator (“fiber-opticrosette system”) is a few grams in weight, displaces a 0.2 mm×2 mm×1 cmvolume, and allows for response times measured in milliseconds.Moreover, such a fiber-optic rosette may have sufficient immunity toelectromagnetic interference (“EMI”) while providing high sensitivity tostrain and stress wave signals over a wide frequency range withoutsignificant resonant response. Such a fiber-optic rosette system canprovide high rate data recording, such as approximately up to a 2kilohertz (“kHz”) sampling rate per channel at sub-microstrainresolution. Such a fiber-optic rosette system may be used for monitoringstructures under static and dynamic loading conditions.

FIG. 1 is a block diagram depicting an exemplary fiber-optic strainsystem 100. Fiber-optic strain (“FOS”) system 100 may be configured toconvert optical signals from FBG fiber-optic sensors 111 in a rosette orrosette-like pattern into analog voltage outputs that may be directlyinterfaced with strain instrumentation. FOS system 100 may include asubsystem, namely fiber-optic system for sensing (“fiber-optic sensingsystem”) 119.

Along those lines, an example FBG interrogation system may havebroadband super-luminescent light-emitting diodes (“SLED”), an opticalcirculator, and a spectral engine 112, such as for example an InGaAsspectral engine. A commercially available spectral engine 112 mayinclude a Volume Phase Grating (“VPG”) for a spectral element 107 and amulti-element array detector 108, such as for example an InGaAs CCD(charge-coupled device) array detector. In such system, an opticalsignal reflected back from FBG fiber-optic sensors 111 may be spectrallydispersed with such a VPG, such that reflected power from each VPG mayextend over only a few pixels of a CCD array detector. This reflectedpower extending to just a few pixels may represent a sufficient numberof data points for a downstream digital signal processor or digitalsignal processing algorithms of a general-purpose processor to beapplied to provide higher resolutions. Using a multi-element arraydetector facilitates high-speed parallel processing, real time spectruminspection with sub-millisecond (“sub-ms”) response times, and sub-picometer (“sub-pm”) spectral resolution.

Strain-induced wavelength shifts experienced by one or more of FBGfiber-optic sensors 111 may be converted to analog voltage signals(“analog output”) 102 that resemble parametric outputs of a conventionalstrain gauge signal conditioner. This allows an FOS system 100 to workas a high-performance “drop-in” replacement for a signal conditioner inconventional strain measurement systems, as described below inadditional detail.

FOS system 100 may include an FBG Analyzer (“FBGA”) module 110, aSystem-on-Chip (“SoC”) module 120, FBG fiber-optic sensors 111 in anoptical fiber 101, an optical fiber-to-structure bonding material 104,and a digital-to-analog (“D/A”) converter 130. Optionally, FOS system100 may be coupled to a network 190, which may include the Internet orother network, for Cloud storage/computing 191. Optionally, one or moreweb-browser enabled devices 192 may be used to communicate with suchCloud storage/computing 191 via such network 190.

FBG fiber-optic sensors 111 in optical fiber 101 housing may be coupledto receive and provide an optical signal via such optical fiber 101, theformer of which may be for optical transmission of light from abroadband light source 105. A bonding material 104 may be used to coupleoptical fiber 101 having FBG fiber-optic sensors 111, namely bonding FBGfiber-optic sensors 111 to a material or structure under test 103. Acyanoacrylate or other adhesive may be used for example for a bondingmaterial 104. Optionally, rather than direct bonding to a material orstructure under test 103, a polymer substrate may be used as describedbelow in additional detail.

A broadband light source 105, such as an LED light source, including anSLED, of FBGA module 110 may provide light to an optical circulator 106of FBGA module 110, and such light may be sent through to optical fiber101 via passing through optical circulator 106 through to FBGfiber-optic sensors 111. Responsive to strain-induced wavelength shiftsexperienced by one or more of FBG fiber-optic sensors 111, reflectedlight from FBG fiber-optic sensors 111 may be provided as returnedoptical signals via optical fiber 101 to optical circulator 106 forspectral element 107 of FBGA module 110. As described below inadditional detail, more than one optical fiber 101 may be coupled toprovide a rosette or rosette-like pattern of FBG fiber-optic sensors111, which may be coupled to a spectral element 107 through opticalcirculator 106.

Reflected light may be spectrally dispersed through spectral element107, which in this example is one or more VPGs of a spectral engine 112of FBGA module 110. Such dispersed light may be detected by a photodiodearray 108 of FBGA module 110, which in this example is an InGaAsphotodiode array; however, other types of photodiode arrays may be usedin other implementations. As described above, a spectral engine 112 mayinclude a CCD array detector 108. Along those lines, a CCD arraydetector 108 may include or perform the function of an analog-to-digitalconverter (“A/D”) converter 109 as part of spectral engine 112. Inanother example, a separate A/D converter 109 may be used, as either ananalog or digital array detector 108 may be used. For purposes ofclarity and not limitation, it shall be assumed that a photodiode array108 is used that does not incorporate an A/D converter 109.

Data output of photodiode array 108 may be digitized using an A/Dconverter 109 of FBGA module 110 to provide digital data output 113.Digital data output 113 of A/D converter 109 may be packetized by anon-board integrated circuit packetizer 139 of FBGA module 110, which inthis example is separate from an FPGA of SoC module 120. However, inanother implementation, packetizer 139 and/or A/D converter 109 may beimplemented in an FPGA of SoC module 120. Such packetized informationmay be forwarded from packetizer 139 to SoC module 120 forpost-processing. Packetizer 139 may be for hardwired Ethernet or otherhardwired communication, or packetizer 139 may be for wirelesscommunication, such as for WiFi, WLAN, or other wireless traffic.Moreover, even though a single channel system is illustrativelydepicted, a multi-channel system may be implemented as described belowin additional detail.

SoC module 120 may include a CPU complex 121, a programmable gate arraydevice 122, and main memory 123. In this example, such programmable gatearray device 122 is an FPGA; however, in other implementations, othertypes of integrated circuits, whether programmable gate array devices ornot, may be used to provide a digital-to-analog (“D/A”) interface 124and digital signal processing (“DSP”) hardware 125.

CPU complex 121, which may be on a same FPGA as D/A interface 124 andDSP hardware 125 in another implementation, in this implementationincludes a dual-core CPU 127 running firmware 126. However, a singlecore or other types of multi-core CPUs may be used in otherimplementations. Generally, a signal conversion block 150, which may bein CPU complex 121, may include a peak detector 129, a Web socket 136,firmware stored in memory (“firmware”) 126, and a spectral powerconverter 131. Firmware 126 may receive data from packetizer 139 of FBGA110 into a ring buffer 128 of SoC module 120, which may also be of CPUcomplex 121.

Ring buffer 128 may be used to store a continuous stream of samples frompacketizer 139 of FBGA 110 to in effect allow FOS system 100 to plotoutputs of FBG fiber-optic sensors 111 in a rosette or rosette-likepattern over a period of time. Firmware 126 may be configured to cleanup data from ring buffer 128. Data from ring buffer 128 may be providedto a peak detector 129, and detected peaks may be provided from peakdetector 129 to spectral power block 131 to quantify spectral powerassociated with each of such peaks detected. Along those lines, firmware126 may quantify wavelength shifts, which are directly proportional tothe amount of strain experienced and spectral power sensed by each ofFBG fiber-optic sensor 111 in a rosette or rosette-like pattern. Thispost-processed data 132 may be stored in main memory 123.

In this implementation, programmable gate array 122, which is coupled tomain memory 123, is configured to provide hardware that reads data 132that firmware 126 has placed in main memory 123 and that performs signalprocessing tasks on such data 132 using DSP hardware 125. An example ofa signal processing task may be an FFT and/or the like to measure anyvibration components in data 132.

Output of DSP hardware 125, such as an FFT output for example, may bewritten back to main memory 123 as data 133 for use by CPU complex 121.D/A interface 124 of programmable gate array device 122 may be used tosend FBG sensor data 132 to an external D/A converter 130 to mimic aparametric output of a conventional strain signal conditioner. FOSsystem 100 may work as a high-performance “drop-in” replacement for asignal conditioner in conventional strain measurement systems, and soanalog output 102 may be provided to conventional strain measurementinstrumentation (not shown). Each output of D/A converter 130 mayrepresent an output of one FBG fiber-optic rosette sensor of FBGfiber-optic sensors 111.

CPU complex 121 via firmware 126 may be configured to read strain data133 that programmable gate array 122 has placed in main memory 123. CPUcomplex 121 may optionally include either or both an Ethernet interface134 or a USB WiFi interface 135 to forward strain data 133 to one ormore remote computers connected over network 190. Optionally, anexternal Cloud server or servers for providing Cloud storage/computing191 may take outputs from multiple FOS systems 100 and store them in adatabase for further analysis by software running on such Cloudserver(s). In this example, such computers may include multipleHTML5-compliant Web browsers 192 to communicate with such Cloud serversand/or to communicate with one or more FOS systems 100 to access straindata 133, which may allow users to make business decisions and/orconfigure individual FOS systems 100 using corresponding optional Websockets 136 of CPU complexes 121 of such systems. However, in another orthis implementation data may be wirelessly transferred, using forexample a TCP/IP protocol, to a remote or local notebook computer fordata analysis.

For portability, SoC module 120 and one or more fiber-optic sensingsystems 119 may be powered with a battery-based power supply (“battery”)138, which in this example may be a 5 volt battery power supply. Alongthose lines, because of an ability to multiplex data from FBG sensors,more than one fiber-optic sensing system 119 may optionally be coupledto a same SoC module 120. Such one or more fiber-optic sensing systems119 may be coupled to a same ring buffer 128, such as using USB or othertype of communication connection. However, for purposes of clarity andnot limitation, only one-to-one relationship between fiber-optic sensingsystem 119 and SoC module 120 is described below.

FIG. 2 is a block diagram depicting an exemplary FOS system 200. FOSsystem 200 is the same as FOS system 100 of FIG. 1, except SoC module120 is replaced with a network interface 201, and functions associatedwith SoC module 120 may be performed by Cloud storage/computing 191.

Fiber-optic sensing system 119 may optionally be a standalone system,such as without an interrogator node directly mechanically coupledthereto. Along those lines, a smaller battery power supply (“battery”)518 than battery 138 may be used to power fiber-optic sensing system119.

Network interface 201 may be an Ethernet or other type of networkinterface configured for hardwired and/or wireless communication with anetwork 190. In this example, network interface 201 is for wirelesslycommunicating with network 190 for sending packetized data frompacketizer 139 to network 190 for Cloud storage/computing 191. Alongthose lines, data sourced from FBG fiber-optic sensors 111 may be storedand processed remotely with respect to FBGA module 110. Processingoperations, including associated processing functions, provided with SoCmodule 120 may be provided with Cloud storage/computing 191. Moreover,network interface 201 may be configured to include packetizer 139,namely network interface 201A. Network interface 201A may beincorporated into FBGA module 110 or may be separate therefrom.

To recapitulate, multifunctional integrated FBG fiber-optic sensors 111may be coupled to an FBGA module 110, as part of an interrogationsystem. FBG fiber-optic rosettes as described herein can be interrogatedby a compact, wireless, battery powered fiber-optic interrogator forboth strain and AE detection, such as SoC module 120. Optionally oradditionally, such interrogation system may be remotely provided usingCloud storage/computing 191. A fiber-optic rosette array may be fielddeployed with a multichannel wireless interrogator node to allow data tobe remotely recorded, analyzed, and displayed for visualization andlarge scale damage prognostics. Along those lines, a fiber-optic rosettearray, such as of FBG fiber-optic sensors 111, may be used with Cloudstorage/computing 191 for storage and/or analysis of such data obtained.

In the former example implementation, FBG fiber-optic sensors 111 may beused with a miniaturized interrogation device, including a stand-alonecompact multichannel fiber-optic strain/AE interrogator with wirelessdata acquisition capability for strain and damage monitoring. In thelatter example implementation, FBG fiber-optic sensors 111 may be usedwith only an FBGA module 110 of such a miniaturized interrogation devicewith wired and/or wireless data acquisition capability for strain anddamage monitoring.

The latter example implementation facilitates wide spread deployment ofFBG fiber-optic sensors 111 coupled to an FBGA module 110. Such FBGfiber-optic sensors 111 and FBGA module 110 systems may be compact andlow power. Such FBG fiber-optic sensors 111 and FBGA module 110 systemsmay be deployed with only battery-based power systems and wirelessconnectivity. Moreover, use of a broadband light source for collecting asignal for strain measurement may reduce cost, size and powerconsumption in comparison with a narrowband light source.

FIG. 3 is a block diagram depicting an exemplary rosette or rosette-likepattern 310 for FBG fiber-optic sensors 111 of FIGS. 1 and 2. FBGfiber-optic sensors 111 include FGB sensor R1 or 301, FBG sensor R2 or302 and FBG sensor R3 or 303. Each FBG sensor 301 through 303 mayinclude a fiber grating array of sensors or fiber grating sensor array.Optionally, FBG sensors 301 through 303 may be attached to an uppersurface of substrate 313, such as a polymer substrate for example toprovide a “patch.” A lower surface of substrate 313 may be bonded,adhered, glued or otherwise attached to surface 308. For example, apatch substrate 313 may have a single FBG sensor or more than one FGBsensor in a rosette or rosette-like pattern 310 on one side and apeel-off backing for exposing an adhesive for attachment on an oppositeside. Optionally, rather than surface mounting of FBG fiber-opticsensors 111 to an upper surface of substrate 313, FBG fiber-opticsensors 111 may be integrated into a substrate 313 structure.

Optionally, in or outside of plane strain pattern 310 may an FBGfiber-optic temperature sensor 304. For temperature compensation due toroom temperature shift, a FBG temperature sensor 304 may be used tomeasure ambient temperature, and temperature values obtain using FBGtemperature sensor 304 may be used to offset contribution from thermalstrain due to room temperature change.

In order to determine the three independent components of plane strain,two normal strains ε_(x) and ε_(y) and shear strain ε_(xy), namely threelinearly independent strain gage measurements, may be used. Along thoselines, FBG sensor 301 may be positioned aligned to an x-axis 311 toobtain an ε_(x) measurement for horizontal normal strain in thex-direction, and FBG sensor 302 may be positioned aligned to a y-axis312 to obtain an ε_(y) measurement for vertical normal strain in they-direction. FBG sensor 303 may be positioned at an angle between FBGsensors R1 and R2 to obtain an ε_(xy) measurement for shear strain inthe xy-direction.

FBG sensors R1 through R4 may be bonded or otherwise adhered to asurface 308 of a structure 300 to be monitored for strain, or moreparticularly plane strain for surface 308 generally a plane. Optionally,structure 300 may be monitored for temperature too, such as previouslydescribed. For loading applied at an angle θ 309 with respect to x-axis311 or FBG sensor R1 301, plane strain may be measured for structure300.

In the presence of only a two-dimensional strain field, namely N=2, astrain response R may be expressed as a sum of each principal strainvector ε multiplied or dot product by a direction vector r of a straingage. This strain response R relationship may be mathematicallyexpressed as:

$R = {\sum\limits_{i = 1}^{2}{\overset{\rightarrow}{ɛ} \cdot \overset{\rightarrow}{r}}}$

From the strain response R mathematical relationship above, for arectangular rosette, directionally normal strain (“normal strain”) inany direction on a surface 308 may be related to two principal strainsε₁ and ε₂ with the latter normal or perpendicular to the former and anangle θ 309 of loading from a principal axis to a direction of strain,namely a direction of a principal strain ε₁. The relationship betweensuch two principal normal strains from three measurements R₁ through R₃of strain by FBG sensors R1 through R3 respectively may bemathematically expressed as:

$ɛ_{1,2} = {\frac{R_{1} + R_{2}}{2} \pm {\frac{1}{\sqrt{2}}\sqrt{\left( {R_{1} - R_{3}} \right)^{2} + \left( {R_{2} - R_{3}} \right)^{2}}}}$for R1, R2, R3 respectively corresponding strain measurements from FBGsensors R1, R2 and R3. A loading angle θ, namely a direction of strainfor a structure 300 under loading, may be mathematically related to R1,R2, R3 strain measurements associated therewith. This relationshipbetween loading angle θ and R1, R2, R3 strain measurements may bemathematically expressed as:

$\theta = {\frac{1}{2}{\tan^{- 1}\left( \frac{R_{1} + R_{2} - {2R_{3}}}{R_{1} - R_{2}} \right)}}$

Though an exemplary rosette or rosette-like pattern 310 for FBGfiber-optic sensors 111 was illustratively depicted with three FBGsensors 301 through 303, in another implementation any two of FBGsensors 301 through 303 may be used for a rosette or rosette-likepattern 310, which in this example is for a first quadrant partialrosette-like pattern 310. For example, either principal normal strainFBG sensor 301 or 302 along with a shear strain FBG sensor 303 may beused; or principal normal strain FBG sensors 301 and 302 may be used.However, a less reliable total strain measured may result in using onlytwo of such FBG sensors.

FIG. 4 is a block diagram depicting an exemplary wireless network 400.Wireless network 400 is further described with simultaneous reference toFIGS. 1 through 4.

Wireless network 400 may include a plurality of fiber-optic sensingsystems 119. Such fiber-optic sensing systems 119 may have FBGfiber-optic sensors 111 attached to structures, which in this exampleare oil storage tanks 401 with an adhesive or bonding material 104.However, other structures may be used. Moreover, fiber-optic sensingsystems 119 may be attached using a magnet and/or an bonding material104.

Antennas 402, such as may be part of a wireless network interfaces 201,may be used to communicate information sourced from FBG fiber-opticsensors 111 to a wireless router 404. Wireless router 404, which mayinclude a modem for communication to the Internet or other network, mayoptionally be hardwired to an Ethernet or other communication cable 405,as a hardwired backhaul for communication information to Cloudstorage/computing 191. Wireless router 404 may be separately poweredwith respect to fiber-optic sensing systems 119.

Along those lines, fiber-optic sensing systems 119 may be enclosed. Ahousing 406 may enclose fiber-optic sensing systems 119, as well ascover FBG fiber-optic sensors 111 attached to oil storage tanks 401.Because FBG fiber-optic sensors 111 do not provide an electricalsparking ignition source, a small battery 518 used to power fiber-opticsensing systems 119 and optical radiation from a broadband light source105, namely possible sources of ignition, are both encased in a housingto reduce any possibility of electrical sparking ignition. Optionally,battery 518 may include a photovoltaic array coupled to charge/rechargebattery 518 to extend time between maintenance of fiber-optic sensingsystems 119.

Wireless router 404 may optionally be positioned on top of a tower 403,which tower 403 is separately located away from oil storage tanks 401.Having wireless router 404 positioned remotely from oil storage tanks401 may be less prone to damage, theft, tampering, and source ofelectrical sparking ignition with respect to an oil storage tank 401.

Accordingly, fiber-optic sensing systems 119 may obtain strain data, aswell as optionally temperature data, and wirelessly communicate suchinformation to wireless router 404. Optionally, fiber-optic sensingsystems 119 may be used for AE sensing, as described below in additionaldetail, and thus damage data obtained by AE sensing may likewise bewirelessly communicated to wireless router 404.

Wireless router 404 may be part of a base station 404 for cellularcommunication with or without a backhaul 405. Along those lines, networkinterface 201 may include a one-way cellular radio for communication tobase station 404. In another implementation, network interface 201 mayinclude a two-way cellular radio for communication to and from basestate 404. Moreover, in a wireless local area network or WLANimplementation, network interface 201 may be configured for two-waycommunication with wireless router 404. In a two-way communicationimplementation, fiber-optic sensing systems 119 may be remotely checkedand maintained for proper operation, in addition to having strain datacommunicated to Cloud storage/computing 191 for storage and processing.

In another implementation, wireless router 404 may be a wireless accesspoint 404. Along those lines, hardwired communications cable 405 may becoupled to a router, hub or switch in a known manner.

In yet another implementation, fiber-optic sensing systems 119 may beBluetooth devices configured for communicating with a Bluetooth accessserver 404. Moreover, a multi-radio access server and accesspoint/router 404 may be used for a combination of Bluetooth and WiFi forexample.

For example for dynamic strain monitoring of a portion of an oil storagetank 401, broadband light from a SLED may be directed to a FBG-basedstrain rosette. Reflected light may be dispersed by a dispersive mediumand then such dispersed-reflected light may be detected by a CCD array.Such CCD array may digitize such detected dynamic strain data. In animplementation, this digitized dynamic strain data may be locallystored, such as by using a field-programmable-gate-array (“FPGA”) and anSoC, such as for example a Zedboard SoC. In an implementation, suchdynamic strain data may be wirelessly transferred, including to a localbase station laptop or notebook computer, for further analysis. Wirelessor wired transfer may use the TCP/IP protocol or another protocol.

It should be understood that the amount of data communicated formonitoring 24-hours a day, seven days a week (“24-7 monitoring”) is notan excessive amount. Therefore, low bandwidth and correspondingly lowpower implementations may be used without negatively impactingmonitoring capability.

FIG. 5 is a flow diagram depicting an exemplary fiber-optic strainsensing flow 100. Fiber-optic strain sensing flow 100 of FIG. 5 isfurther described with simultaneous reference to FIGS. 1 through 5.

At operation 501, FBG fiber-optic sensors 111 are attached in or as arosette or rosette-like pattern 310 to a structure 401. At operation502, a light signal is generated with a broadband light source 105.

At operation 503, such light signal generated by broadband light source105 may be received by an optical circulator 106. At operation 504, suchlight signal may be provided from optical circulator 106 via opticalfiber 101 to FBG fiber-optic sensors 111.

At operation 505, a returned optical signal from FBG fiber-optic sensors111 may be received by optical circulator 106 via optical fiber 101. Atoperation 506, such returned optical signal may be provided from opticalcirculator 106 to a spectral engine 112 to provide an output signal.Such returned optical signal may include peaks associated with planestrain obtained by at least one first Fiber Bragg grating sensor of FBGfiber-optic sensors 111 having either a directionally normal x-axisposition or a directionally normal y-axis position and by a second FiberBragg grating sensor of FBG fiber-optic sensors 111 having an x-yposition.

Optionally, at 506, a preprocessing operation at 509 may be used. Alongthose lines, raw data of a returned optical signal may be preprocessed,such as with an SoC as described herein or other IC device, of an edgenode having a broadband light source such as for example fiber-opticsensing system 119 of a Cloud connected system, to provide such anoutput signal. Such preprocessing may be used to reduce bandwidth usedin transmitting raw data and/or to increase overall system processingcapability, which may be used to enhance speed of analytics processingby a Cloud-based computing system. Optionally, such raw data may be sentwith or without preprocessed results obtained from edge node processingof such raw data. Such preprocessed raw data may be for extracting dataassociated with features of such raw data to obtain a subset of such rawdata. An FPGA-based or other microcontroller may be configured to sendraw, processed, and/or feature extracted data from such an FPGA-basedSoC to a Cloud-based computing system for archival, analytics,functions, and/or visualization.

At operation 507, such output signal may be packetized by a networkinterface 201. At operation 508, such output signal may be communicated,via hardwire or over-the-air wireless connection, to a network for Cloudstorage and processing, as previously described.

Using a low-power SHM with FBG rosettes to monitor, including withoutlimitation to continuously monitor, for damage precursors, such as forexample principal strain direction changes, a higher power AE sensorinterrogation may be activated responsive to detection of strain fromsuch monitoring. As previously described, low-power SHM using FBGrosettes may be used to continuously monitor for changes in a hoststructure's principal strain direction. Detection of strain by such aFOS system 100 or 200 may suggest damage. Along those lines, detectionof strain by an FOS system 100 or 200 may be used to automaticallytrigger a higher power AE sensor to provide for better characterizationof such suspected damage.

Unlike traditional “always on” AE platforms, having an FOS systemdetection trigger an AE platform may use less power. Furthermore,wireless communication between an FOS system and an AE platform, namelybetween different sensing platforms, may be used to further support anInternet of Things (“IoT”) implementation. For example, a combination offiber-optic sensor rosettes for strain monitoring and a fiber-opticsensor for acoustic emissions monitoring may be attached to a structure,and may be used to sample data in an area and to monitor crackinitiation and propagation.

Along those lines, passive principal strain direction monitoring may beused as a damage initiation trigger for one or more active sensingelements, including without limitation for example acoustic emissionssensors. Along those lines, AE sensors can be combined with one or moreFOS systems to provide for damage location, and such AE sensors can bepowered on-demand, periodically, and/or responsive to an FOS systemdetection to further establish reliability while preserving an energyefficient implementation.

Generally, IoT involves integration of physical objects and wirelessnetworks by instrumenting these objects with sensors and actuators. IoTtechnology has gained attraction in various industries such astransportation, health care, military, security, manufacturing, and manyothers. Infrastructure health monitoring is one of the areas where IoTcan significantly increase accuracy and accessibility that conventionalapproaches fail to provide. IoT technology may combine environmental andphysical parameter sensing with data transmission and processing, suchas through wireless communication techniques or wireless sensornetworks, providing an effective platform to combine SHM, ConditionBased Monitoring (“CBM”), and/or Prognostic Health Management (“PHM”).

Low-cost and low power fiber optic sensors may be used for continuousmonitoring with real-time data collection and analysis. Once sensor datais collected, processed, transmitted, and analyzed, structural damagecan be successfully detected or predicted. Generally, long-term healthmonitoring of structures as described herein has capability foreffective structural management, predictive maintenance and/or safeoperation.

As described below in additional detail, FOS systems 100 or 200 may becombined with AE interrogation. For example, an FOS system 100 or 200may use an SLED for a light source, and AE interrogation node may use adistributed feedback (“DFB”) laser for a light source. Both lightsources and associated signal conditioning hardware may be integratedinto a single module to provide combined AE and dynamic strainmeasurements.

For AE detection, light from a DFB laser may be sent to a FBG-based AEtransducer (“FBG transducer”). Light reflected from such FBG transducermay be detected by a photodetector, such as an InGaAs photodetector.Light converted by such photodetector into a demodulated analogelectrical signal may be digitized, such as using an A/D converter of adata acquisition module or board. For example, with digital trackingcontrol, such a DFB laser may be continuously locked to a mid-reflectionwavelength of such an FBG transducer for ultrasonic wave detection. Sucha demodulated photodetector signal may be interfaced to a compact,high-speed data acquisition board for signal processing and storage. AEextracted data may be wirelessly sent to a base station laptop fordisplay and analysis, such as using a ZigBee protocol for example.

AE detection may also be used for strain measurements with asub-microstrain resolution, namely generally involving minimallypico-meter wavelength detection sensitivity. Detection of such a smallwavelength shift may be obscured by environmental and system noise. Toeffectively remove a noise contribution, the output signal of aphotodetector may be fed into a digital feedback controller, which inturn provides a feedback signal for laser control electronics. Thislaser tracking scheme allows a laser wavelength to be continuouslylocked to a stable point at a mid-reflection wavelength of a Bragggrating to produce a high signal-to-noise ratio for providing a directstrain measurement via a generated output error signal. Due toout-of-band noise rejection by a feedback controller, a resultingsignal-to-noise ratio may be enhanced, permitting sub-microstraindetection sensitivity. Once a laser is locked, a DC strain signal isgenerally stable, and laser wavelength may be highly resistant toenvironmental noise that tends to move a laser wavelength away from astable point to which it is locked. This system stability may provideboth improved signal-to-noise strain measurements and reliable AC strainand ultrasonic wave detection.

Optionally, as described below in additional detail, all or a portion ofan FOS systems 100 or 200 may be combined with AE interrogation toprovide a wavelength tracking system. For example, an FOS system 100 or200 may use an SLED for a light source, and AE interrogation node mayuse one or more lasers, such as a distributed feedback (“DFB”) laser forexample, for a light source. Both light sources and associated signalconditioning hardware may be integrated into a single module to providecombined AE and dynamic strain measurements with dynamic wavelengthtracking. Such tracking may be used to tune a laser wavelengthresponsive to a shift in an FBG wavelength. This shift may for examplebe due to damage to a structure.

FIG. 6 is a perspective side view depicting an exemplary optical fiber101 and a corresponding reflected signal 610. Optical fiber 101 may havea fiber core 601. Fiber core 601 may include FBG fiber-optic sensors 111inscribed on single-mode photosensitive silica-based fibers, such asGe/B doped silica fibers for example. Input edge-to-input edge spacing602 between neighboring FBG fiber-optic sensors 111 may be Λ, andrefractive indexes η of FBG fiber-optic sensors 111 may be differentfrom one another, such as η₀ through η₃ for example, for forming a Bragggrating.

A sensor rosette patch, such as previously described, may be coupled toa miniaturized interrogation device including a stand-alone compactmultichannel fiber optic strain interrogator with optional wireless dataacquisition. Optionally, such interrogator may be coupled to an energyharvesting device. Such a fiber-optic sensor may be: only a few grams inweight (e.g., 10 grams or less for sensor weight); compact (e.g., 0.2mm×2 mm×1 cm or smaller); responsive (e.g., a response time of measuredin a few milliseconds or less); immune to EM interference; and highlysensitive to strain and stress wave signals from DC to ultrasonicfrequency with flat (e.g., no resonance) response. Such a sensor may bepassive, so as not to consume power. Such a sensor size may bemultiplexed along length of an optical fiber 101. Such a sensor may havean AE resolution of sub-nanostrain and a load resolution ofsub-microstrain.

A core refractive index of fiber bore 601 may have a digital clock-likepattern corresponding to FBG fiber-optic sensors 111 and spacings 602.Spectral response of an input signal may be different than spectralresponse of a transmitted signal, and spectral response of a transmittedsignal may be different than spectral response, such as spectralresponse 610 for example, of a reflected signal.

With respect to a spectral response 610 of a reflected signal from FBGfiber-optic sensors 111, under an unstrained or baseline condition foroptical fiber 101, such reflected spectral response 610 may appear as animpulse 603 centered at a Bragg wavelength 604 or λ_(B). However, undera strained or non-baseline condition for optical fiber 101, suchreflected spectral response may appear as an impulse centered at awavelength other than a Bragg wavelength, such as generally indicated byan example impulse 605 shifted in wavelength away from impulse 603. Thiswavelength shift may be detected and correlated to strain causing suchwavelength shift.

Discrete sensing using FBG fiber-optic sensors 111 as point sensors,with a laser or broadband light source, may be implemented to measurestrain, temperature, pressure, vibration, and/or AE. However,distributed sensing using FBG fiber-optic sensors 111 may be implementedusing a pulse laser light source and generally an entire sensing lengthof an optical fiber 101, such as a silica fiber (e.g., a pure silicafiber) as a sensing medium. Such distributed sensing may be used tomeasure strain, distributed temperature sensed (“DTS”), and/ordistributed acoustics sensed (“DAS”).

FIG. 7-1 is a block diagram of a perspective view depicting an exemplarypatch 700 attached to a host structure 401. FIG. 7-2 is a block diagramof a cross-sectional view along A-A of patch 700 of FIG. 7-1. Withsimultaneous reference to FIGS. 1 through 7-2, patch 700 is furtherdescribed. Patch 700 is an example of a substrate 313.

Patch 700 may have a single FBG sensor or more than one FGB sensor. Oneor more patches 700 may be used to provide a rosette or rosette-likepattern 310. A rosette or rosette-like pattern 310 may be formed byhaving one or more optical fibers 101 coupled and/or taped down to abacking 705.

In this example, backing 705 may be formed using one or more flexiblethin films with an FBG baseline strain after embedding. Backing 705thickness, width, and/or length may be adjusted according to anapplication. For example, a bond ply used in flexible printed circuitindustry or other suitable flexible substrates may be use together withepoxies to bond an entire FBG sensor rosette pattern 310, including thegrating and non-grating fiber sections of three optical fibers 101. Thuseven though only one optical fiber 101 is illustratively shown forpurposes of clarity, it should be understood that more than one opticalfiber 101 may be attached to a patch 700. Moreover, an input end 701 ofa section of optical fiber 101 may be used for receiving a transmittedoptical signal, and an output end 702 of such section of optical fiber101 may be used for reflecting back a reflected optical signalassociated with such transmitted optical signal.

Patch 700 may be bonded directly with a bonding material 104 to asurface of a host structure 401 to be monitored. A bonding material 104layer may be used to couple backing 705 to host structure 401.

An encapsulation material suitable for use with FBG sensors mayoptionally be used for a stronger strain coupling to a host structure tobe monitored. Such encapsulation material may be for additionalprotection from environmental forces and/or structural integrity. Inthis example, an encapsulation layer 703 is used to encapsulate asection of optical fiber 101, including at least the entirety of an FBGsensor thereof. Optionally, an adhesive film 704 may be used to couple asection of optical fiber 101 to backing 705 prior to encapsulation byencapsulation layer 703.

For example, Dupont Kapton® polyimide film backing from CS Hyde Companycoupled with Henkel-Adhesives Hysol® 9696 adhesive film may beimplemented optionally with a polyurethane protective film layer forencapsulation. Hysol® film parameters published by the vendor include athickness of 0.001 in. and a tensile strength of 30 ksi. A polyurethaneprotective film layer can be thermoformed around a section of opticalfiber 101 to provide protection from environmental factors whileminimizing any effects on structural coupling of one or more FBGfiber-optic sensors 111 to a host structure 401.

Geometry may be optimized for maximum strain transfer from a hoststructure to FBG fiber-optic sensors 111. For example, positioning ofpatch edges with respect to FBG fiber-optic sensors 111 may be used toreduce or minimize edge effects at the location of FBG fiber-opticsensors 111. Accordingly, distances d1, d2, and d3 from outer edges ofoptical fiber 101 to corresponding outer edges of encapsulation layer703 or backing layer 705, as applicable, may be adjusted to reduce orminimize edge effects.

Thickness and material properties of a backing and one or more adhesivelayers may impact the transfer of strain. Along those lines, backing 705may be expanded to allow integration of FBG fiber-optic sensors 111 toform a rosette or rosette-like pattern 310 on a single patch. Once ageometric arrangement for FBG fiber-optic sensors 111 of a patch 700 isdetermined, such as by finite element analysis, at least two FBGfiber-optic sensors 111 may be encapsulated in a rosette or rosette-likepattern 310. In the above example of FIG. 3, three FBG fiber-opticsensors 111 are encapsulated for a patch 700.

For example, to precisely control the orientation of FBG fiber-opticsensors 111 to create a rosette pattern 310, a stencil may be created,such as by laser cutting a thin sheet of plastic in a desired geometryfor example. Adhesive may be placed within stenciled areas, and FBGfiber-optic sensors 111 may be set in place in such stenciled areas.Such stencil may be removed while adhesive is curing. A protectiveencapsulation layer may be applied, and a heat gun may be used toprovide sufficient temperature to thermoform such protectiveencapsulation layer to such patch assembly, including eliminating anyair-gaps.

With respect to AE and strain, FIG. 8 is a block diagram depicting anexemplary AE and FOS system 800. In this example, there are N, for N apositive integer greater than one, FBG fiber-optic sensors 111-1 through111-N coupled through corresponding optical multiplexers or opticalcirculators 801-1 through 801-N to a multi-channel fiber optic voltageconditioning (“FOVC”) 810M and a multi-channel FBG analyzer 110M. Eventhough a multi-channel FOVC 810M and a multi-channel FBG analyzer 110Mare described, a single-channel FOVC 810 and a single-channel FBGanalyzer 110 may be used in another implementation as follows from themore complex description of a multi-channel implementation.

Multi-channel FOVC 810M and multi-channel FBG analyzer 110M may beoperated out-of-phase with respect to one another in order to share FBGfiber-optic sensors 111-1 through 111-N. For example, multi-channel FOVC810M and multi-channel FBG analyzer 110M may be operated or active onopposite states of a sync control signal 811 coupled to multi-channelFOVC 810M and multi-channel FBG analyzer 110M.

A detailed description of a multi-channel FOVC may be found in U.S.patent application Ser. No. 14/814,355. However, this or another type ofmulti-channel FOVC compatible with an FBG sensor may be used.

On a timing interval of control signal 811 for which multi-channel FOVC810M is to be active, multi-channel FOVC 810M may transmit signals toFBG fiber-optic sensors 111-1 through 111-N to receive correspondingreturned signals. Multi-channel FOVC 810M may process such returnedsignals to provide ADC converted voltage analog outputs as digital dataoutputs 805-1 through 805-N corresponding to such returned signals. Suchdigital data outputs 805-1 through 805-N may be provided to a datamultiplexer (“mux”) 809.

Control signal 811 may optionally be used for providing a control selectto mux 809 for selecting either digital data outputs 805-1 through 805-Nor digital data outputs 113-1 through 113-N for an interval of a cycleof control signal 811. Along those lines, a 50-50 duty cycle or otherduty cycle commensurate with timing of operations of multi-channel FPVC810M and multi-channel FBG analyzer 110M may be used.

On a timing interval of control signal 811 for which multi-channel FBGanalyzer 110M is to be active, multi-channel FBG analyzer 110M maytransmit signals to FBG fiber-optic sensors 111-1 through 111-N toreceive corresponding returned signals. Multi-channel FBG analyzer 110Mmay process such returned signals to provide ADC converted voltageanalog outputs as digital data outputs 113-1 through 113-N correspondingto such returned signals. Such digital data outputs 113-1 through 113-Nmay be provided to a mux 809.

A selected output of mux 809 may be provided to a packetizer 111, whichmay be time-slice shared by multi-channel FPVC 810M and multi-channelFBG analyzer 110M for wireless transmission of data, such as previouslydescribed and not repeated here for purposes of clarity and notlimitation.

FIG. 9-1 is a block diagram depicting exemplary AE and FOS system 800 ofFIG. 8 optionally coupled to a Composite Overwrapped Pressure Vessel(“COPV”) 840. Though a COPV 840 is used for a structure 401, anothertype of structure may be used. Furthermore, a structure other than atank or vessel may be used. For example, a pipe or other conduit may bemonitored for leaking, cracking or other damage. Moreover, a structuremay be a power station or other large power transformer, and AE and FOSsystem 800 may be for monitoring for partial discharge and/or physicaldamage.

In this example, three FBG fiber-optic sensors 111-1 through 111-3 arecoupled to COPV 840 in a rosette or rosette-like pattern 310. However,fewer or more than three FBG fiber-optic sensors may be used in otherimplementations.

FBG fiber-optic sensors 111-1 through 111-3 may be corresponding coupledto three optical multiplexers or optical circulators 801-1 through801-N. Thus, COPV 840 may be monitored for leaking, cracking, or otherconditions indicating current or potential damage or tampering.Optionally, COPV 840 may be monitored for temperature with an FBGfiber-optic sensor, as previously described.

FIG. 9-2 is a block diagram depicting exemplary AE and FOS system 800 ofFIG. 8 coupled to COPV 840. However, in this example, FBG fiber-opticsensors 111-1 through 111-3 are not shared as between multi-channel FPVC810M and multi-channel FBG analyzer 110M. Rather, a separate FBGfiber-optic sensor 111-4, which is not part of a rosette or rosette-likepattern 310, is separately directly coupled to COPV 840 for directcommunication with multi-channel FPVC 810M. Along those lines, opticalmultiplexers or optical circulators 801-1 through 801-3 used for sharingFBG fiber-optic sensors may be omitted in this example.

In this example, three FBG fiber-optic sensors 111-1 through 111-3 arecoupled to COPV 840 in a rosette or rosette-like pattern 310. FBGfiber-optic sensors 111-1 through 111-3 may be corresponding coupled tothree channels of multi-channel FBG analyzer 110M.

Thus, COPV 840 may be monitored for leaking, cracking, or otherconditions indicating current or potential damage or tampering.Optionally, COPV 840 may be monitored for temperature with an FBGfiber-optic sensor, as previously described.

FIG. 10-1 is a block diagram depicting an exemplary single-channel lasertracking-based fiber optic voltage conditioning system 850. In order toprovide voltage conditioning for acoustic emissions (“AEs”)measurements, an open/closed loop control 851 of FOVC 810 of fiber opticvoltage conditioning system 850 may be used to actively track a laser toBragg wavelength of an FBG fiber-optic sensor 111.

A function of laser tracking-based FOVC 810 may be to provide fiberoptic voltage conditioning of returned optical signal 803 coupled to FBGsensors (collectively and singly “FBG sensor”) 111 via a fiber opticcable or line, as described below in additional detail. Along thoselines, a multifunction fiber optic sensor, such as fiber Bragg gratingsensor, may be used. For AE measurements, using a fiber Bragg gratingsensor in a fiber optic housing, an entire sensing area can be bondeddirectly to a surface of a structure (“measurement surface”) such aswith a permanent epoxy or other bonding material. The direct bonding ofa bare fiber to a measurement surface may provide a high-level of stresswave coupling from such measurement surface to a sensing area.

However, over time such bonding material may result in local stressalong such Bragg grating sensor, such as along a grating length forexample, and this may lead to creation of an unstable optical cavityformed by multiple gratings, such as along such grating length forexample. These optical cavities may negatively affect FOAE sensorperformance by changing the slope of a Bragg reflection spectrum and/orincreasing Fabry-Perot noise. To address this issue, an overhangingridge configuration may be used, where a grating under tension may beused for hanging over two stand-offs bonded to a structure's surface toaddress the above-described problem, such as for AE and/or strainsensing. Moreover, for an overhanging ridge configuration, a grating maybe used for hanging over shear wave-coupling gel, on either or bothsides thereof, to provide a flexible bonding to a structure's surface toaddress the above-described problem, such as for AE and/or temperaturesensing. Optionally, a package sensor multiplexing two or more FBGsensors on the same package with different Bragg wavelengths installedcan be multiplexed in serial or parallel. Such a package sensor may beused for AE, strain, and/or temperature sensing. Moreover, optionally amultiplexed-multi-sensing package sensor may be used. For example, twoor more different types of sensors (e.g., AE/strain and AE/temperature)can be multiplexed on the same package for multi-sensing.

Optionally, another function of such laser tracking-based FOVC 810 maybe to convert returned optical signal 803 from FBG sensor 111 to anoptional AC voltage analog output 855 that may directly interface withconventional AE instrumentation, which is illustratively depicted asoptional AE data acquisition system 830. An optional analog signaloutput 855 of FOVC 810 may resemble that of a piezo-electric sensor usedin conventional AE measurements, facilitating sensor “drop inreplacement”.

Along those lines, conventional piezo-electric sensors/preamplifiersignal conditioners can be completely replaced with a combination of FBGsensor 111 and FOVC 810 as described herein without changing existing AEsoftware and electronics of conventional AE data acquisition systems,such as AE data acquisition system 830 for example, leading tosignificant cost saving through minimizing additional hardware/softwareinstallation. In other words, a high-frequency, high-gain photodetectoroutput 805 carrying a high frequency signal may be interfaced directlyto an analog input of a conventional AE data acquisition system 830,such as a Mistras PCI-2 DAQ board from Mistras Group, Inc. of PrincetonJunction, N.J., for example.

Light 804, which may be from a compact, commercially availabledistributed feedback (“DFB”) laser or other laser 882, may be passed viaan optical circulator 883 to an FBG sensor 111. A returned opticalsignal 803 from FBG sensor 111 passing through optical circulator 883may be routed to a photodetector 884. Current control 816, which may beseparate from or part of DFB laser 882, may be initially set at amidrange value between a lasing threshold and a maximum current limit,and a thermoelectric cooler (“TEC”) control 885, which may be separatefrom or part of DFB laser 882, may be tuned to move laser wavelength toa mid-reflection point (“V REF”) of a Bragg wavelength of FBG sensor111.

With laser current and TEC control voltages settled at initial setpoints, namely V TEC SET and V CUR SET 821, laser wavelength may belocked to a mid-reflection point of FBG sensor 111 using simultaneousTEC and current tracking through a closed loop 851proportional-integral-derivative (“PID”) feedback control.

For adjustment for real-time laser tracking control, a chip-based dataacquisition (“DAQ”) board 820, such as an FPGA-based or otherSystem-on-Chip-based (“SoC-based”) circuit board, may be used to recordat least one of a low-gain and/or low-frequency of photodetector outputsignal 806, namely as associated with an analog input toanalog-to-digital converter (“ADC”) 817 and to generate a PID controlsignal 807 of DAC 818.

By “low-gain” and “low-frequency”, it is generally meant an analogoutput signal being below both a threshold gain and a thresholdfrequency, respectively, where such thresholds represent an externalenvironmental change and/or perturbation, including without limitation achange in one or more of temperature, strain, pressure, and/or stress ofa structure under test, including a structure being monitored, as sensedby one or more FGB sensors coupled to one or more optical fibers.Accordingly, such thresholds may vary from application-to-applicationdepending upon the type of structure being tested, as well as use ofsuch structure.

An analog output signal 806 of photodetector 884 may be converted to adigital data output signal 805 by ADC 817, and a digital output of ADC817 may additionally feed an input of a voltage set and store block 819.A digital output of voltage set and store block 819 may feed an input ofDAC 818 to provide an analog PID control signal 807. PID control signal807 may include a PID current error (“CUR ERR”) signal and a PID TECerror (“ERR”) signal. PID control signal 807 may be output from DAC 818.

For purposes of clarity by way of example and not limitation, it shallbe assumed that an FPGA is used to set and store voltages; however, inanother implementation another type of SoC may be used, includingwithout limitation an ASSP, ASIC, or other IC. Along those lines,voltage set and store block (“FPGA”) 819 may be used to set and store alaser current voltage and a TEC control voltage, namely V TEC SET and VCUR SET 821, and FPGA 819 may be used to store a mid-reflection point VREF of a Bragg wavelength of FBG sensor 111. FPGA 819 may be coupled toreceive a digital output 823 from ADC 817, where such digital output 823is a conversion of an analog photodetector output signal 806. FPGA 819may be configured to generate and store TEC and laser current errorvoltages, namely a V TEC ERR and a V CUR ERR, using such digital output823 received. FPGA 819 may provide a digital PID control signal 824,where such digital PID control signal includes a V CUR ERR signal and aV TEC ERR signal, to DAC 818, and DAC 818 may convert such digital PIDcontrol signal 824, namely a V TEC ERR signal and a V CUR ERR signal, toanalog PID control signal 807 having analog PID CUR ERR and PID TEC ERRsignals.

PID CUR and TEC error signals from PID control 807 may becorrespondingly added to CUR and TEC set voltages 821 by adder 822, andrespective sums 808 output from adder 822 may be fed into a controller850 for adjustment of control information provided to laser 882. In thisexample, for purposes of clarity and not limitation controller 850 isbroken out into three controllers or modules, namely a laser controller853 coupled to a current controller 816 and a TEC controller 885.However, in another implementation, current controller 816 and/or TECcontroller 885 may be part of a laser, such as DFB laser 882 forexample. Sums 808 may be used together to compensate for drift of aBragg wavelength, such as due to external environmental changes and/orperturbation including without limitation changes in one or more oftemperature, strain, pressure, and/or stress, by actively tuning laserwavelength responsive to such current and TEC control. Even though a DFBlaser 882 is used, in another implementation a broadband light source orother type of light source may be used.

In this example, both laser TEC and current are used simultaneously tocompensate for FBG wavelength drift from DC up to approximately 20 kHzfor photodetector output signal 806. TEC tracking may be provided bychanging temperature of DFB laser 882 via TEC control 885 to compensatefor FBG sensor 111 wavelength drift caused by environmental changes.While providing large dynamic range, such as for example approximatelyseveral thousand microstrains for strain monitoring, TEC compensationmay be slow, with a maximum response time in the order of seconds orlonger.

In this example, fast, such as for example a few Hz to 20 kHz or higher,real time compensation may not be possible with TEC tracking. Alongthose lines, laser current compensation, as described herein, may beused with a much higher response time, possibly up to approximately 20kHz or higher, subject to limitations of response time of electronics oflaser controller 853, and such laser current compensation may be usedsimultaneously with TEC tracking. Tracking by changing laser injectioncurrent may cause changes in both laser wavelengths and intensity,although with much more limited dynamic range, such as for exampleapproximately several hundred microstrains for dynamic strain tracking.For larger dynamic strain monitoring, such as more than approximately athousand microstrains, commercially available distributed Braggreflector (“DBR”) lasers can be used in place of DFB lasers. However, itshould be appreciated that using TEC and current tracking in combinationprovides extended dynamic range and fast response for laser tracking.

Long-distance AE measurement using a laser-based FBG interrogation maybe subject to presence of high amounts of optical noise associated withthe Fabry-Perot effect generated by an optical cavity created by two ormore reflective mirrors. By “interrogation,” it is generally meantproviding a light signal to an optical sensor coupled to a material orstructure under test and obtaining a light signal in return from suchoptical sensor to obtain information therefrom regarding such materialor structure under test. A Bragg grating itself may be considered ahighly reflective mirror. In the presence of another reflective surfacefrom an optical component, such as for example an optical circulator ora scattering center such as a local defect present in a long opticalfiber, unstable, unwanted constructive optical interferences can begenerated due to laser coherence. Accordingly such interferences maycontribute to increased AE background noise, and as a consequence cansignificantly reduce a signal-to-noise ratio (“SNR”) in AE measurements.

To suppress this optical noise, a combination of circulators and opticalisolators between reflection and/or scattering surfaces may be used toprovide unidirectional optical paths and avoid bidirectional opticalpaths between any two reflective optical components, such as describedbelow in additional detail.

FIG. 10-2 is a block diagram depicting an exemplary multichannel FOVCsystem 850. Multichannel FOVC system 850 is further described withsimultaneous reference to FIGS. 10-1 and 10-2.

In multichannel FOVC system 850, an N-channel FOVC 810M is respectivelycoupled to FBG sensors 111-1 through 111-N via corresponding opticalcirculators 883-1 through 883-N, for N a positive integer greater thanone. FBG sensors 111-1 through 111-N may be respective discrete FOAEsensors or an array thereof.

DFB1 through DFBN lasers 882-1 through 882-N and correspondingphotodetectors (“PD”) PD1 884-1 through PDN 884-N may be respectivelycoupled to optical circulators 883-1 through 883-N. Each of DFB lasers882-1 through 882-N may deliver corresponding laser lights 804-1 through804-N respectively into FBG sensors 111-1 through 111-N viacorresponding circulators 883-1 through 883-N. Circulators 883-1 through883-N may then be used to pass corresponding returned optical signals803-1 through 803-N respectively from sensors 111-1 through 111-N on aper channel basis. Returned optical signals 803-1 through 803-N may berespectively provided onto photodetectors 884-1 through 884-N.

Each of the outputs of photodetectors 884-1 through 884-N, which may beimplemented in an example implementation as photodiodes (“PD”) PD1through PDN, may be split into two sections, namely analog signals 806-1through 806-N and optionally analog signals 805-1 through 805-N. Onegroup, namely a low frequency signal output group of signals 806-1through 806-N, may be input into an analog input interface 852, such asrespective analog input ports for example, of an FPGA-based dataacquisition system 820 for laser tracking control generation aspreviously described herein. Another optional group, namely a highfrequency signal output group of analog signals 805-1 through 805-N, mayoptionally be input to a conventional multichannel AE DAQ system 830 forAE measurement.

Even though an FPGA 819 is used as described herein for DAQ 820, anothertype of SoC, an ASSP, an ASIC, or other VLSI type of integrated circuitdevice may be used instead of FPGA 819. However, for purposes of clarityand not limitation, it shall be assumed that an FPGA 819 is used.Furthermore, DAC 820 may exist in a single integrated circuit device,whether such device is a monolithic integrated circuit or an integratedcircuit formed of two or more integrated circuit dies packaged together.

An FPGA 819 may have sufficient resources for integration of one or moreADCs 817, one or more DACs 818, and/or one or more adders 822 thereinfor providing a multichannel FOVC 810M. However, an FPGA may lacksufficient analog resources, and so a separate analog chip, such as forproviding digital-to-analog conversions, may be used. However, forpurposes of clarity by way of example and not limitation, it shall beassumed that FPGA 819 has a sufficient number of ADCs 817 for convertinganalog signals 806-1 through 806-N into corresponding digital dataoutputs 805-1 through 805-N. FPGA 819 may include a digital interface859 for outputting digital data outputs 805-1 through 805-N. Optionally,FPGA 819 may be configured with a control signal generator circuit togenerate and output a control signal 811.

In this example, FOVC 810M includes a DAQ 820 having an FPGA 819configured for inputs 1 through N of an analog input interface 852(“inputs 852”) and outputs 1 through 2N of an analog output interface856 (“outputs 856”), for example separate analog output ports. Inputs852 may correspond to a group of signals 806-1 through 806-N. Pairs ofoutputs 808-1 through 808-N of outputs 302 may respectively be providedto laser controllers 853-1 through 853-N. Laser controllers 853-1through 853-N may provide respective pairs of TEC and current controlsignals 885-1, 816-1 through 885-N, 816-N to DFBs 882-1 through 882-N,respectively.

For purposes of clarity by way of example and not limitation, lasercontrollers 853-1 through 853-N are illustratively depicted as includingcorresponding pairs of current and TEC controllers, which wereillustratively depicted as separate controllers 816 and 885,respectively, in FIG. 10-1 for purposes of clarity. However, it shouldbe understood that controllers 885 and 816 may be incorporated into alaser controller 853.

For purposes of scaling an FOVC 810, it should be appreciated that asingle FPGA 819 may be used by a DAQ 820 configured to support Nchannels. In this example, FOVC 810M does not include opticalcirculators 883-1 through 883-N; however, in another configuration, FOVC810M may include optical circulators 883-1 through 883-N.

Generally, FPGA 819 generates respective sets, such as pairs forexample, of TEC and current control signals 808-1 through 808-N viaanalog output ports 856 of DAQ 820, and such respective sets of TEC andcurrent control signals 808-1 through 808-N may be used to providecorresponding pairs of TEC control and current control signals 885-1,816-1 through 885-N, 816-N to respectively lock DFB lasers 882-1 through882-N to their respective FBG sensors 111-1 through 111-N by addingrespective error signals. Such respective error signals may be generatedfrom FPGA 819 generated PID control to provide current and TEC setpoints via digital summing as previously described herein, though on aper-channel basis in this example of FOVC 810M. For long distancemeasurements, N fiber extenders, whether all transmission fiberextenders, all reflection fiber extenders, or a combination thereof maybe used in conjunction with multichannel FOVC 810M.

FIG. 11 is a block diagram depicting an exemplary self-poweredcontrol/detection system 900. Control/detection system 900 follows fromthe above description of FOVC 810M, and thus is not described inunnecessary detail to avoid repetition.

Control/detection system 900 may be coupled to FBG sensors 111 by fiberoptic lines to control/detection electronics section 930.Control/detection electronics section 930 may include a light sourcecontroller 931, a light source 932, a photo detector 933, and a signalconditioner 933. An analog output from DAC 916 may be provided to lightsource controller 931. Light source controller 931 may be coupled tolight source 932 for control thereof responsive to such analog output.

Light output from light source 932 may be provided to FBG sensors 111for providing an optical input to photodetector 933. Output ofphotodetector 933 may be provided to ADC 917 and to signal conditioner933. Signal conditioner 933 may be for AE.

In this example, a single control/detection electronics section 930 isillustratively depicted. However, in other examples, more than onecontrol/detection electronics section 930 may be present.

Output of signal conditioner 933 may be provided as an input toAE/strain node 921 of a conventional AE/strain system 920. A graphicaluser interface (“GUI”) 922, such as in Java for example, may be used tocommunicate with AE/strain node 921. AE/strain node 921 may be a IEEE1284 compliant node.

AE/strain system 920 may be coupled to a wireless interface 925, such asfor a ZigBee protocol for example, for wireless communication viaantenna 926. For example, a notebook computer 940 may be used tocommunicate with wireless interface 925.

DAC 916 and ADC 917 may be of a data acquisition module or DAQ 918. DAC916 and ADC 917 may be coupled to a closed loop tracking system 913 of atracking communication section 910. Tracking communication section 910may be implemented with an FPGA for example.

Closed loop tracking system 913 may include a power management system914. Power management system 914 may be coupled to a power section 909.

Power section 909 may include a renewable energy input source 901, suchas solar, wind, and/or other an energy harvester for example, a voltageconditioner 902, and a DC-DC converter 904. Renewable energy fromrenewable energy input source 901 may be provided to voltage conditioner902 for conditioning. Voltage conditioner 902 may include a rectifier903, such as a full bridge rectifier for example, to rectify inputvoltage for such conditioning.

Conditioned voltage output by voltage conditioner 902 may be input to aDC-DC converter 904 for conversion to a desired DC voltage level from aDC input voltage. DC-DC converter 904 may include a battery charger 905to charge a battery or other storage device.

Output DC voltage from DC-DC converter 904 may be provided to aclosed-loop tracking system 913, such as previously described herein.Closed-loop tracking system 913 may include a power management system914 for managing power.

Data may be communicated between DAC 916 and ADC 917 and closed-looptracking system 913, as previously described herein. Moreover, data maybe communicated between data communication system 912 and closed-looptracking system 913 both of which may be of tracking/communicationsection 910. Data may be communicated between command communicationsystem 911 and data communication system 912 both of which may be oftracking/communication section 910. Again, as examples of such systemshave been previously described, such description is not repeated forpurposes of clarity and not limitation.

Data, including without limitation command information, may becommunicated between systems 911 and/or 912 and a graphical userinterface (“GUI”) 922. GUI 922 may be in communication with an AE/strainnode 921 both of which may be of AE/strain system 920.

AE/strain system 920 may be coupled to wireless interface 925, which maybe configured for a ZigBee protocol for example, for wirelesscommunication via one or more antennas 926. Along those lines, anotebook computer 940 may be used for communication withcontrol/detection system 900 via wireless interface 925. Notebookcomputer 940 may be a conventional notebook PC or other computingdevice, and such computing device may be programmed with special-purposesoftware for control/detection system 900.

In an example, power electronics design of control/detection system 900may be based on the MB39C811 chip from Spansion, Inc. This chip isspecialized in harvesting vibration energy. Piezo patches harvestmechanical vibration energy into electrical energy originally in ACwhich may be converted into DC. To enhance harvesting energy, a voltageconditioner may double any low AC voltage before being rectified by ahigh efficient bridge rectifier and filtered into DC power.

In this example, a DC-DC converter may convert raw DC power depending onvibration source into a stable 5V supply to all electronics. Smartbattery charging circuitry under micro-computer control may charge abattery for backup when vibration energy sources cease.

A BM39C811 device could provide a load of 100 mA at 5V with theefficiency up to 90% for a large range of input voltage from 8V to 20Vharvested from vibration energy. For AE wireless monitoring, a MistrasGroup's commercially available 1284 AE wireless node may be used. Thisnode contains multiple sensing elements, on-board signal processingelements, signal conditioning electronics, power management circuitry, awireless data transmission element and is capable of using an energyharvesting unit. For this device, the main sensing elements are acousticemission sensors; however, several parametric inputs are provided toconnect to different sensors such as strain gage, thermometer, and pHmeter. The output of all these sensors may be combined and analyzed at asensor node in order to reduce a data transmission rate and consequentlyreduce power consumption.

For this example, the total power may be as low as 18 Watts for theentire instrument when in full operation. Total power consumption can besignificantly reduced to below 10 Watts by operating at 50% laser powerand by designing sensor wavelengths under nominal load to overlap withlaser wavelength at room temperature, minimizing laser thermalcompensation provided by a thermos-electric cooling system. In astandard operation mode, a microcontroller (“MCU”) can be preprogrammedto initiate a data collection process at a fixed frequency and duration.For example, an internal timer of an MCU can turn on sensors for a fewseconds per hour during a load measurement mode. For power transformermonitoring applications when energy harvesting is desirable, a UBI-2590lithium ion rechargeable battery can be used, which can provide up to a12 A-hr at 14.4V, equivalent to 173 W-hrs DC. Using such a battery, thisexample FOAE instrument may last for up to 20 hours without energyharvesting. For more extended operation, shorter acquisition times canbe used. For example, if a data acquisition interval is 1 second perhour or less, this example FOAE instrument can last more than one yearwithout recharging the battery.

For example, a compact four channel wireless AE detection systemincluding a FBG sensor array, four miniaturized laser controllers, fourphotodiodes, a 4-channel DSP-based data acquisition system combined witha four-channel wireless AE node may be implemented. An example 4-channelFOAE system may include: (1) a power conditioner section for EMIfiltering, conditioning and managing the system power from the energyharvesting unit or from a battery, (2) a control/detection electronicssection including 4 DFB laser controllers, 4 detector boards, and AE andstrain signal conditioner circuits, (3) a tracking data acquisitionsection including a 4-channel digital DSP/FPGA-based data acquisitionboard, and (4) an AE/strain data acquisition system including acommercial PAC 1284 wireless AE/strain wireless node (“1284 node”). Forthis example, an analog laser controller/photodetector board may includelasers and control electronics, InGaAs photodiodes and preamplifiercircuits, and signal conditioning electronics which provides bothhigh-frequency AE signals and low frequency strain signals to a 1284node. In such a DSP/FPGA board, a DSP provides closed loop computationfor tracking while an FPGA performs communication management amongchannels and to the outside world. Such an FPGA may deliver calculatedanalysis results from such a DSP in a digital format to controlwavelength of individual laser sources, such as laser diodes, via twoDACs, one for laser wavelength tracking and the other for laser opticalpower. For AE and strain data acquisition, a 1284 node may accept highbandwidth AE analog signals (10-250 kHz) from a detector amplifier andlow frequency strain signals (DC to 5 kHz) from voltage conditionedlaser control signals. A summary of AE features and real-time strainsignals may be transmitted wirelessly to a nearby laptop via a WLANtransceiver, and such data may be plotted in real-time. Both raw AEwaveform and strain data may be stored locally in such a 1284 node, suchas using a SD memory card.

Supervised Wavelength Tracking

FIG. 12 is a block diagram depicting an exemplary wavelength trackingsystem 1000 using fiber optic voltage conditioning, as described above,for sensing AE. Wavelength tracking system 1000 uses a broadband lightsource system (“BLS system”) 1022, such as previously described formeasuring microstrain. BLS system 1022 provides a monitoring subsystemfor wavelength tracking system 1000 to provide an initial FBG positionfor FOAE tracking recovery. Along those lines, BLS system 1022 may havemultiple wavelengths within a BLS. For example, each color in broadbandlight from a broadband light source represents a different wavelength.Wavelength multiplexing may be used to feed information to a controller1021. Controller 1021 may be implemented to include a finite statemachine (“FSM”). In an example implementation, controller 1021 may beimplemented with a microcontroller. For purposes of clarity by way ofexample and not limitation, controller 1021 shall be assumed to be amicrocontroller 1021. Microcontroller 1021 may be a trusted “supervisormicrocontroller”, and microcontroller 1021 may be used to prompt BLSsystem 1022 to feed information for a particular wavelength for trackingrecovery.

One or more lasers may be used with BLS system 1022 for dynamicconfiguration or reconfiguration. In order to provide voltageconditioning for acoustic emissions (“AE”) measurements, an open/closedloop control system may be used to actively track a wavelength of anarrowband tunable light source, such as a distributed feedback (“DFB”)laser, to a Bragg wavelength of a fiber Bragg grating (“FBG”) sensor. Awavelength tracking-based fiber optic acoustic emission voltageconditioner (“FOVC”) may be used to convert an optical signal from FBGsensors to an AC voltage output that can directly interface withconventional AE instrumentation. The signal output of such an FOVCresembles that of a piezo-electric sensor used in convention AEmeasurements, facilitating sensor “drop in replacement”.

However, AE with laser tracking is sensitive, and thus more likely tolose tracking. To quickly recover tracking or moving to anotherwavelength, a laser may be tuned to lock to a different FBG wavelengthof one of FBG sensors 111. For example, if there is a “hot spot”indicated by microstrain measurements obtained from BLS system 1022, oneor more lasers may be directed to such “hot spot” using dynamicreconfiguration by dynamically retuning such one or more lasers to lockto one or more corresponding different FBG wavelengths. For example,when a rosette principal direction drifts from a nominal valueindicating presence of damage, one or more lasers may be tuned to one ormore FBG wavelengths associated with such rosette. Independently or inaddition to retuning for microstrain detected damage, one or more lasersof an AE data acquisition system may be locked to one or more FBGwavelengths for a “wake up” mode and/or when a load exceeds a loadthreshold.

Different wavelengths may be in a rosette including different FBGwavelengths. Moreover, a BLS light may share a fiber with a laser sourcelight, where a BLS and a laser use same or different wavelengths, as alaser source can overwhelm a BLS source. Furthermore, with different FBGwavelengths, lasers may share a same fiber.

Wavelength tracking system 1000 includes a SoC 1020, which optionallymay be used for edge computing. SoC 1020, such as may be an FPGA withone or more FPGA-based microprocessors, which may be coupled to acloud-based computing system 1009 through a network interface 1003 ofsuch SoC 1020. Such a cloud-based computing system 1009 may include datastorage 1006, such as may include database (“DB”) software, and one ormore processors 1007 configured with analytics software, such as SaaSfor example, for processing raw and feature extracted data obtained fromSoC 1020. Optionally, such SoC 1020 may be configured for localprocessing of raw data, such as for extraction of information from suchraw data, in an optional FPGA-based edge computing configuration. Alongthose lines, SoC 1020 may send results and/or information obtained fromlocal processing of raw data, rather than, or optionally in addition to,sending raw data in order to conserve bandwidth and/or enhance overallprocessing capacity. One or more web browser computers or otherweb-configured devices 1008 may be put in communication with cloud-basedcomputing system 1009 for accessing stored data, whether raw and/orprocessed data.

SoC 1020, in addition to network interface 1003, may include a lasercontroller 1001, memory 1002, and one or more electronic dataacquisition (“eDAQ”) controllers, such as eDAQ controllers 1005 forexample. Laser controller 1001 may be configured with laser controlsoftware 1004.

Laser controller 1001 and network interface 1003 may be coupled throughand share memory 1002. Additionally, eDAQ controllers 1005 may becoupled to and share memory 1002. While two eDAQ controllers 1005 areillustratively depicted, in another example fewer or more than two eDAQcontrollers 1005 may be used.

A supervisor microcontroller 1021 may be coupled through bus 1024 to SoC1020 for communication therewith. However, in another implementation,supervisor microcontroller 1021 may be incorporated into SoC 1020.Memory 1002 may be configured with a look-up table 1025. Look-up table1025 may include predetermined settings for different FBG wavelengthsfor one or more eDAQ controllers 1005, eDAQ converters 1010, laserdrivers 1013, photodetectors 1017, and/or other components of wavelengthtracking system 1000. Such settings in lookup table 10125 may be usedfor tuning one or more modules of wavelength tracking system 1000 foroptimum performance for a variety of applications.

To provide supervised wavelength tracking, a multichannel sensorinterrogation array may be used. Different and/or same types oflogistics sensors 1023, such as a GPS sensor, an accelerometer sensor,and/or a liquid monitoring sensor, among other types of sensors, may beused for logistics for an AE sensing application. Additionally oroptionally, logistics sensors 1023 may include one or more controllersfor pulse generators. Such one or more controllers for pulse generatorsmay be configured to generate predefined periodic pulses of stress wavesfor individual automated sensor testing (“AST”). These stress waves maybe used to provide acousto-ultrasonic testing to capture a principalstrain and a principal direction of each sensor rosette using pitchcatch and/or pulse-echo processing. Along those lines, for example apiezo-electric actuator may be placed remotely from or proximate tosupervisor microcontroller 1021, and a controller for pulse generationmay be in wireless communication with such an actuator for causing suchactuator to pulse for propagation of such pulse along a monitoredstructure. Such a controller for pulse generation and/or other logisticssensors 1023 may be coupled to supervisor microcontroller 1021 forobtaining different sensing inputs associated with logistics,multi-sensing data collection and/or sensor fusion analytics, which mayinclude one or more of finite state machine states including wake-upcall triggering, system initialization, wavelength tuning, trackingcontrol, wavelength monitoring, tracking recovery, system shut-down, anddynamic re-configuration for on-demand new sensor selection.

A BLS system 1022 may be coupled to supervisor microcontroller 1021 forproviding wavelength tracking. A front panel and user interface 1013 maybe coupled through bus 1024 to SoC 1020 and supervisor microcontroller1024 for unlocking those components, if locked. Along those lines, SoC1020 and supervisor microcontroller 1024 may be protected with bootprotection, which may be hardware and/or software based protection.Additionally, front panel and user interface 1013 may be coupled throughbus 1024 to SoC 1020 and supervisor microcontroller 1024 forinitializing and monitoring status of those components.

In a trusted mode of operation, a trusted supervisor microcontroller1024 may be responsible for unlocking and initializing hardwaresub-modules. With a trusted boot of supervisor microcontroller 1024, SoC1020, an array of laser drivers 1014, and an array of photodetectors(“PDs”) 1016 may be unlocked and initialized under control of supervisormicrocontroller 1024. A locked state may be present at power-up or resetof wavelength tracking system 1000. A look-up table 1026 in memory 1002,or in memory of microcontroller 1024 in another implementation, may bepreloaded with specific information to prohibit functioning ofwavelength tracking system 1000 responsive to a failure ofauthentication of one or more module components of such system. In otherwords, authentication may be performed at initialization to validate asystem configuration of wavelength tracking system 1000.

Additionally, supervisor microcontroller 1024 may be configured tomonitor system statistics of wavelength tracking system 1000 to ensureproper operation, such as operation within predefined parameters forexample. If a module or other portion of wavelength tracking system 1000is not operating within predefined parameters, supervisormicrocontroller 1024 may halt or shut down such module or other portionnot operating within predefined parameters.

Supervisor microcontroller 1024 may additionally provide “black box”functionality by allowing an external device to view logged events,which may be periodically logged by supervisor microcontroller 1024.Such an event log may be used to view a state of wavelength trackingsystem 1000, including an ability to determine for example whether ananomalous event has occurred. Supervisor microcontroller 1024 maycommunicate with front panel and user interface 103 to illuminate statusLEDs or other devices used to communicate status to allow a user to haveup-to-date information about operation of wavelength tracking system1000.

Supervisor microcontroller 1024 may be used to offload system managementtasks from SoC 1020 from real-time control processes running on SoC1024, which in this example is implemented with an FPGA. These real-timecontrol processes may be used for controlling eDAQ converters 1010.

Crack growth and/or damage hot spot monitoring using an FBG rosette maybe used to distinguish between damage and its effect on principal straindirection and/or changes in loading by comparing with neighboringrosettes. Principal strain direction at one or more FBG rosettes mayshift in the presence of a local crack for example, while one or morenearest neighbor FBG rosettes may not indicate any change. Thus, one ormore FBG rosettes touching or proximate to such local damage may be usedto indicate damage.

Along those lines, in an example for a local crack or other detectabledamage, the principal direction of a rosette touching or proximate to acrack may show change independent of temperature, geometry or any otherbackground change. In addition to being independent of environmentalconditions, a nearest neighbor damage index may be baseline free, whichmay be useful in applications with more or less arbitrary loadingconditions. In other words, no pristine or other background informationneed be used, such as measured and subtracted out, because localinformation may be directly compared to global information. In otherwords, one or more rosettes local to a damage site may be directlycompared to one or more nearest neighbors absent a baseline. Such adirect comparison may be used to avoid false positives, as comparisonwith nearest neighbor rosettes may be used to indicated damage withoutbeing confounded by load direction and/or load magnitude. Any change inload direction may change principal strain vectors at all rosettes in anarray; however, with nearest neighbor comparisons, local damage may onlychange the principal strain vector at such local location.

For a nearest neighbor comparison, damage severity may be quantified byusing a normalized damage index (“DI”) formula as follows:

${DI} = {\sin{{\theta_{i} - {\frac{1}{n}{\sum\limits_{k = 1}^{n}\theta_{k}}}}}}$where i is a rosette immediately at or adjacent to a damage instance,such as a crack for example, and k are one or more nearest neighborrosettes. The larger the difference between i and k may indicated alarger amount of damage.

For a grid or array of rosettes, both near field and far field effectsof loading may be detected. A damage index value for DI may range from 0to 1, inclusive, with 0 being no crack and with 1 being for a rosettedirectly on a crack. Along those lines, a DI value may be compared orotherwise processed against other DI values of other rosettes in a gridto identify a damage region, or at least a probability of a damageregion relative to such DI value. However, as indicated above, abaseline-free DI value is an absolute value for an FBG, and not arelative value as compared to some existing DI-based damage detectiontechniques that requires knowledge of the pristine condition as abaseline reference for background subtraction. This means that abaseline-free DI value obtained from a local discontinuity in aprincipal direction compared to one or more of its nearest neighbors maygenerally be insensitive to changes in a local environment, boundaryconditions, loading conditions, and may be used to indicate a“permanent” shift in a principal direction due to damage.

Moreover, rosettes may be used to provide damage imaging for real-timevisualization and/or post damage evaluation. Rosettes may be placed oninterior and/or exterior surfaces of a structure to form atwo-dimensional and/or three-dimensional image. Rosettes may be providedin a single layer or multiple layers.

eDAQ controllers 1005 may respectively control eDAQ converters 1010.While two eDAQ converters are illustratively depicted, in anotherimplementation more or fewer eDAQ converts 1010 may be used. Each eDAQconverter 1010 may include an ADC 1011 and a DAC 1012.

eDAQ converters 1010 may be used for providing analog signals to laserdrivers 1013. In this example, there are laser drivers 1013-1 and 1013-2fed by one eDAQ converter 1010, and there are laser drivers 1013-3 and1013-4 fed by another eDAQ converter 1010. Laser drivers 1013 may be setto different FBG wavelengths. So in this example, four differentwavelengths for laser light outputs may be used with analog informationfrom two eDAQ converters 1010.

Outputs from laser drivers 1013 may be provided to respective opticalLEDs of an array of LEDs 1015. Outputs from LEDs 1015 may respectivelybe input to FBG fiber-optic sensors 111-1 through 111-4, with each FBGfiber-optic sensor 111 in a rosette or rosette-like pattern. Outputs forFBG fiber-optic sensors 111-1 through 111-4 may be respectively input tophotodiodes 1017-1 through 1017-4 of an array of photodiodes 1016.Output of each of photodiodes 1017-1 through 1017-4 is corresponding AEinformation 1019-1 through 1019-4. AE information 1019-1 and 1019-2 maybe fed back to a corresponding source eDAQ converter 1010, and AEinformation 1019-3 and 1019-4 may be fed back to another correspondingsource eDAQ converter 1010.

Because BLS may be more easily tracked than laser light and may consumeless power, BLS system 1022 may be used as an initial interrogator ofFBG fiber-optic sensors 111, which may be shared with lasers 1013(though not shown in this FIG. 12 for purposes of clarity and notlimitation). If such interrogation results in detection of damage bymicrostrain information and/or one or more DIs for corresponding FBGfiber-optic sensors 111, lasers 1013 may be used to obtain more preciseAE information 1019. SoC 1020 may be coupled to receive an output signalfrom BLS system 1022 and configured to clean up such output signal,detect peaks in such output signal, and quantify power associated withone or more peaks detected in such output signal. This information fromsuch an output signal may be used by SoC 1020, as SoC may be configuredto quantify wavelength shifts in such output signal as being directlyproportional to amounts of strain and spectral power sensed byfiber-optic sensors in a rosette or rosette-like pattern. Thesewavelength shifts may be used to provide one or more wavelengthadjustment information for one or more wavelength adjustments to shiftone or more laser drivers to corresponding FBG wavelengths of FBGsensors 111 associated with such output indicating a damage area. SuchFBG sensors 111, and thus fiber-optic lines associated therewith, may beshared as between lasers and a BLS, as previously described.

In another implementation, such output signal from BLS system 1022 maybe combined with information output from one or more logistics sensors1023, such as by supervisor microcontroller 1021, for providing to SoC1020, and optionally subsequently to cloud-based computing system 1009,for analysis by either or both SoC 1020 or analytics processors 1007.Analysis results from cloud-based processing at cloud-based computingsystem 1009 may be communicated to SoC 1020. Such analysis results fromeither or both SoC 1020 or analytics processors 1007 may be used toidentify which one or more FBG sensors 111 to target to obtain AEinformation 1019 and to shift one or more wavelengths for one or morelaser drivers 1013, as previously described, associated with suchidentified FBG sensors 111. Again, for example, this shift may be forobtaining AE information 1019 for an identified hot spot.

With renewed reference to FIG. 5, continuing reference to FIG. 12, andadditional reference to a flow diagram of FIG. 13 depicting an exemplarywavelength tracking flow 1030, wavelength tracking flow 1030 is furtherdescribed.

At 1031, logistics information may be detected with at least onelogistics sensor 1023 coupled to a controller 1021 to provide logisticsinformation associated with an output signal obtained at 506 from a BLSsystem 1022. Such output signal obtained at 506 may be provided to SoC1020 for local and/or cloud-based processing. For purposes of clarity byway of example and not limitation, it shall be assumed that cloud-basedprocessing is used, though in other implementations local processingwith an SoC 1020, a locally wirelessly or wired coupled computer, and/orcloud-based processing may be used.

At operation 507A, such output signal and associated logisticsinformation may be packetized by a network interface 201 of SoC 1020. Atoperation 508A, such output signal and associated logistics informationmay be communicated, via hardwire or over-the-air wireless connection,to a network for Cloud storage and processing, as previously described.

In an implementation, using a low-power SHM with FBG rosettes tomonitor, including without limitation to continuously monitor, fordamage precursors, such as for example principal strain directionchanges, a higher power AE sensor interrogation may be activatedresponsive to detection of strain from such monitoring. As previouslydescribed, low-power SHM using FBG rosettes may be used to continuouslymonitor for changes in a host structure's principal strain direction.Detection of strain by such a FOS system 100 or 200 may suggest damage.Along those lines, detection of strain by an FOS system 100 or 200 maybe used to automatically trigger a higher power AE sensor to provide forbetter characterization of such suspected damage. Additionally, damagemay cause a shift in an FBG wavelength of an FBG sensor, and sowavelength tracking off of a BLS may be used in addition to selectiveactivation of AE sensor interrogation.

At 508A analytic processing of an output signal and associated logisticsinformation may be performed by a cloud-based computing system 1009 toobtain at analytics results, which may include wavelength adjustmentinformation. At 1032, such wavelength adjustment information may becommunicated, such as from cloud-based computing system 1009, to beobtained by SoC 1020. Such wavelength adjustment information may beresponsive to such an output signal and associated logistics informationfor a fiber-optic sensor 111 of fiber-optic sensors in a rosette orrosette-like pattern. At 1033, a wavelength of a laser light source,such as a laser driver 1013, may be shifted responsive to suchwavelength adjustment information.

FIG. 14 is a flow diagram depicting an exemplary damage induced anomalydetecting and monitoring flow 1040 for a multi-sensing system, such aspreviously described. Such a multi-sensing system may be power efficientby a broadband light source strain sensing system for triggering use ofa laser-based acoustic emission sensing system, as previously described,where the former system is low power as compared with use of the lattersystem. Damage induced anomaly detecting and monitoring flow 1040 isfurther described with simultaneous reference to FIGS. 1 through 14.

At 1041, a fiber-optic strain sensing system having a broadband lightsource coupled to a controller may be operated, such as previouslydescribed for example. At 1042, a strain and a direction thereof of adamage induced anomaly in a structure may be detected by at least onefirst Fiber Bragg grating sensor of fiber-optic sensors of thefiber-optic strain sensing system. Such strain and direction thereof maybe a principal strain and a principal direction thereof, respectively.

At 1043, a laser-based acoustic-emission sensing system may be activatedor otherwise triggered responsive to the detecting of a damage inducedanomaly by a fiber-optic strain sensing system, such as previouslydescribed. At 1044, wavelength tracking logistics for monitoring such adamage induced anomaly may be provided. Such wavelength logistics mayinclude use of an FSM implemented with a microcontroller, such aspreviously described. Along those lines, a supervisor microcontrollermay be used to provide or supervise provisioning of one or more forms ofwavelength tracking logistics. Such wavelength tracking logistics,including without limitation finite states, may include one or more ofthe following for a of a laser-based system: wake-up call triggering,system initializing, wavelength tuning, controlled wavelength tracking,wavelength monitoring, wavelength recovery, system shut-down, or dynamicreconfiguration for on-demand sensor selection including withoutlimitation changing as between sensors.

At 1045, growth of a damage induced anomaly may be monitored. Thismonitoring may include using wavelength tracking logistics provided at1044. However, more particularly, such monitoring may include operationsat 1046 through 1048.

At 1046, obtained may be a baseline-free damage index value for a damageinduced anomaly detected by at least one first Fiber Bragg gratingsensor of fiber-optic sensors, such as previously described herein. At1047, a first principal direction for a direction associated with suchdamage induced anomaly detected by such at least one first Fiber Bragggrating sensor may be determined. At 1048, such first principaldirection may be compared with a second principal direction detected byat least one second Fiber Bragg grating sensor of such fiber-opticsensors, which may be a nearest neighbor or other neighbor of such theat least one first Fiber Bragg grating sensor, for a direction shift ofsuch first principal direction.

While the foregoing describes exemplary apparatus(es) and/or method(s),other and further examples in accordance with the one or more aspectsdescribed herein may be devised without departing from the scope hereof,which is determined by the claims that follow and equivalents thereof.Claims listing steps do not imply any order of the steps. Trademarks arethe property of their respective owners.

What is claimed is:
 1. A method for a multi-sensing system, comprising:operating a fiber-optic strain sensing system having a broadband lightsource coupled to a controller; detecting a strain and a directionthereof of a damage induced anomaly in a structure by at least one firstFiber Bragg grating sensor of fiber-optic sensors of the fiber-opticstrain sensing system; wherein the strain and the direction thereof is aprincipal strain and a principal direction, respectively, for the damageinduced anomaly; triggering a laser-based acoustic-emission sensingsystem responsive to the detecting of the damage induced anomaly by thefiber-optic strain sensing system; and providing wavelength trackinglogistics for monitoring the damage induced anomaly.
 2. The methodaccording to claim 1, wherein the wavelength tracking logistics areprovided by one or more of wake-up call triggering, wavelength tuning,feedback-controlled wavelength tracking, wavelength recovery tracking,or dynamic sensor selecting.
 3. The method according to claim 1, furthercomprising the monitoring of the damage induced anomaly for growth, themonitoring comprising: obtaining a baseline-free damage index value forthe damage induced anomaly detected by the at least one first FiberBragg grating sensor of the fiber-optic sensors; determining a firstprincipal direction for the principal direction associated with thedamage induced anomaly detected by the at least one first Fiber Bragggrating sensor; and comparing the first principal direction with asecond principal direction detected by at least one second Fiber Bragggrating sensor of the fiber-optic sensors neighboring the at least onefirst Fiber Bragg grating sensor for a direction shift of the firstprincipal direction.
 4. The method according to claim 1, wherein thedetecting comprises: having the fiber-optic sensors of separate opticalfibers in a rosette or rosette-like pattern; and generating a lightsignal with the broadband light source of a Fiber Bragg Grating analyzerfor having Fiber Bragg Grating wavelengths in the rosette orrosette-like pattern in parallel.
 5. The method according to claim 4,wherein the detecting further comprises: receiving the light signal byan optical circulator of the Fiber Bragg Grating analyzer; providing thelight signal from the optical circulator to the fiber-optic sensors;receiving a returned optical signal from the fiber-optic sensors by theoptical circulator; and providing the returned optical signal from theoptical circulator to a spectral engine of the Fiber Bragg Gratinganalyzer to provide an output signal.
 6. The method according to claim5, further comprising: preprocessing raw data at an edge node having thebroadband light source to provide the output signal; packetizing theoutput signal by a network interface; and communicating the outputsignal to a network for storage and analytics processing.
 7. The methodaccording to claim 5, further comprising: detecting logisticsinformation with at least one logistics sensor coupled to a controllerto provide logistics information associated with the output signal;obtaining wavelength adjustment information responsive to the outputsignal and the logistics information for at least one fiber-optic sensorof the fiber-optic sensors in the rosette or rosette-like pattern; andshifting a wavelength of a laser light source responsive to thewavelength adjustment information.
 8. The method according to claim 5,wherein the returned optical signal comprises peaks associated withplane strain obtained by at least one first Fiber Bragg grating sensorof the fiber-optic sensors having either a directionally normal x-axisposition or a directionally normal y-axis position and by a second FiberBragg grating sensor of the fiber-optic sensors having an x-y positionrespectively of different ones of the optical fibers.